urrently underway in one of London’s wealthiest areas, the De Vere Gardens development is located on the site of two recently demolished hotels in Kensington and

Chelsea, next to Hyde Park. The project will provide two new seven-story buildings and one five-story block incorporating retained façades. In the middle of the site, encompassed on three sides by extensive building work (including the enlarge-ment of the existing basement to two levels), is a block of 15 privately owned flats. Sir Robert McAlpine, Ltd., a leading UK building and civil engineering company, is the main contractor on the De Vere project. In this pricey and historical area, moni-toring the buildings and the retained facades for structural damage is a critical part of the contract.

Having realized inadequate results from a monitoring subcon-tractor, McAlpine decided to perform the monitoring itself. The monitoring system had several requirements: It had to be managed in-house; it had to be automated, real-time, reliable and easy to use; and above all, it had to be in place before piling began—a tight timeframe.

“The tight time schedule was significant for us,” explains Senior Land Surveyor Vince Bridle, the manager tasked with installing and overseeing the system. “Understandably, residents were concerned about structural damage to their property. We felt that a comprehensive monitoring program would help alleviate their fears.” The team looked at several systems, ultimately

selecting a real-time solution of two Trimble S8 robotic total stations with Trimble 4D Control software.

“We were already familiar with the hardware and software, which cut down the learning curve,” says Bridle. “And having seen the system demonstrated on a small-scale project, we knew that we could manipulate it for our specific requirements. Strong vendor support to get us up and running was also a factor.”

Moving TargetsA control network of 13 base targets was established on site, and 98 25mm prisms were fixed on the structures to be monitored. The total stations were positioned at opposite ends of the site, one on the McAlpine site building and the other on the highest point of a retained façade wall, where there was sufficient room to power it with three solar panels. A 2.4-GHz wireless radio link connects the total stations to an office PC running the Trimble 4D Control software, where email and text alerts are generated automatically.

Trimble 4D Control software post-processes the monitoring data collected by the total stations. It computes rounds of angles measured in the field as individual sessions and reveals any movement of targets over time. The terrestrial engine in Trimble 4D Control processes each session, validates the data, then stores the point information together with the raw data. Other modules of the software use this data to create warnings and alarms, as well as graphs and reports.